In recent years, individual identification technologies have attracted attention. In these technologies, Identification information is assigned to each object to recognize the history of information on the object and to be utilized in production, management, or the like. In particular, RFID (radio frequency identification) technologies for transmitting and receiving data through wireless communication have been developed in the market. A wireless communication system using RFID technologies is constituted by a wireless communication device (an interrogator) and a data carrier (a transponder), and data is wirelessly communicated between them. The wireless communication device means a device capable of wirelessly transmitting and receiving data, such as a reader/writer, a cellular phone, and a personal computer; in this specification, it is typically referred to as a reader/writer. The data carrier is generally called an RF tag, an ID tag, an IC tag, an IC chip, a wireless tag, an electronic tag, or the like; in this specification, it is typically referred to as an RF tag.
An RF tag includes a variety of circuits needed to wirelessly communicate data with a reader/writer, and one of these circuits is a modulation circuit. The modulation circuit has a function of modulating a carrier wave output from a reader/writer in accordance with a response signal output from a logic circuit, and transmitting a response signal to the reader/writer.
As a method for transmitting a response signal to a reader/writer, a load switching method is known, in which modulation is performed by changing the load impedance between the two terminals of an antenna included in an RF tag (for example, see Patent Document 1).
A modulation circuit using the load switching method includes a switching element and a load. As the switching element, a transistor is generally used, and a resistor is generally used as the load. The transistor and the resistor are connected in series between the two terminals of an antenna. A response signal is input from a logic circuit to a gate of the transistor.
In accordance with the response signal from the logic circuit, the transistor is turned on (brought into conduction) or turned off (brought out of conduction). When a transistor is in an off-state in an ideal modulation circuit, almost no current flows through the modulation circuit and the impedance between the two terminals of the antenna does not change either. Therefore, the potential of a terminal of an antenna is practically not influenced by the modulation circuit. On the other hand, when a transistor is in an on-state in an ideal modulation circuit, a drain current is generated in the transistor; thus, a certain current flows from an antenna to the modulation circuit and the potential of a terminal of the antenna changes. Therefore, the impedance between the two terminals of the antenna also changes. In this manner, in a modulation circuit using the load switching method, a transistor is controlled to be turned on or off in accordance with a response signal from a logic circuit, whereby the impedance between the two terminals of an antenna can be changed and data is transmitted to a reader/writer.
Since an antenna is a balanced device, a sine wave with a certain frequency is input to each of the two terminals of the antenna in an RF tag. A signal at one terminal of an antenna and a signal at the other terminal of the antenna have phases shifted from each other by 180°. However, description is given below on the assumption that the potential of the other terminal of the antenna is fixed (0 V) for the sake of simplicity.